With this chapter eluding to diffuse charge layers, I thought that I would share some insight from my CHEM3013 course of last semester. The following images have been taken from the CHEM3013 lecture slides prepared by Prof. Matt Trau.
In the above image you can see that to a positively charged surface, the anions are the ions that are most concentrated around the surface, while the cations are a bit more scattered, though some are held close to the surface by the surrounding anions. There are also two graphs there, the first depicting the ion concentrations as a function of the distance away from the surface, and the second depicting the potential as a function of the distance.
This is a depiction of what is known as the Stern Layer. In the stern layer, counter-ions adsorb to the surface, and it's their size which determines the stern layer thickness. Since the cations in this case (since the surface is negative) are adsorbed to the surface, and are hydrated, the stern plane is said to be "one hydrated ion radius from the surface"
Finally, to wrap up this blog post, here is the graph from before, but now with the stern layer included in the system. As you can see, the potential drops quite linearly to begin with, then drops linearly.
By the way, in case you want to know the maths of the potential, it's
for the situation without the stern layer, or
for the situation with the stern layer, where psid is the stern potential